Sulfur and chlorine budgets control the ore fertility of arc magmas

Continental arc magmas supply the ore-forming element budget of most globally important porphyry-type ore deposits. However, the processes enabling certain arc segments to preferentially generate giant porphyry deposits remain highly debated. Here we evaluate the large-scale covariation of key ore-forming constituents in this setting by studying silicate melt inclusions in volcanic rocks from a fertile-to-barren segment of the Andean Southern Volcanic Zone (33–40 °S). We show that the north-to-south, fertile-to-barren gradient is characterized by a northward increase in S and Cl concentrations and a simultaneous decrease in Cu. Consequently, we suggest that the concentration of S and Cl rather than the concentration of ore metals regulates magmatic-hydrothermal ore fertility, and that the loss of volatiles prior to arrival in the upper crust impacts ore-forming potential more than magmatic sulfide saturation-related ore metal scavenging.

regime is compressional and favours the development of reverse faults with which the major volcanoes 23 are spatially associated. In the south, the dextral transpressional stress regime is expressed as the Loquiñe-Ofqui Fault Zone, and in this section of the arc both major and minor volcanoes (with locally 25 variable compositions) 9 are typically associated with the major and minor faults of this dominant 26 tectonic feature. Like the geochemistry, this tectonic contrast is also mirrored in the El Teniente-area 27 record wherein a non-contractional Miocene regime associated with relatively thin crust eventually 28 evolves into a contractional Pliocene regime with crustal thickening and periodic eastward arc-front 29 migration 1 . 30 The geochemical and tectonic evidence shows that ore-forming SVZ magmas are associated with arc 31 segments that evolve to compressional stress regimes and thickened crust, which promote prolonged 32 magma storage, differentiation, and homogenization prior to ascent. Sampling volcanoes along the 33 modern SVZ therefore yields insight into long-term trends in arc magmatism which culminate in world-34 class porphyry Cu deposit formation. Contamination of mafic SVZ magma as evidenced by incompatible trace element ratios and time-45 sensitive indicators including U-series disequilibrium and 10 Be/ 9 Be (e.g., refs. 9,10 ) further demonstrate 46 the potential for a locally variable balance of mantle and crustal influences to raise large-ion lithophile element (LILE) concentrations. One likely contaminant source is the residual plutonic material left 48 behind during differentiation and ascent of previous magma batches, which have been shown to 49 interact with some ascending southern SVZ magmas 10 . It is likely that these somewhat older but broadly 50 co-genetic magmas lost a significant fraction of their S and Cl budget during solidification prior to 51 contaminating modern SVZ magma either by separation and loss of late-stage fractional melts and/or by 52 crystallization-induced degassing. Therefore, the partial melts of these rocks assimilated by the modern 53 magmas in newly developed lower-crustal hot zones will be S-and Cl-poor. If this contaminant is 54 genetically similar to the modern magma, such as an ancestral plutonic 'root', it will be difficult to 55 distinguish by traditional Sr-Nd-Pb isotope ratios 2,[9][10][11] . 56 both of which must be satisfied to be considered a pSMI. 62

SUPPLEMENTARY TABLES
One sample was a poorly consolidated coarse lithic tuff, and prepared as mineral separates.     Crosses along the right margin of each plot show representative 1σ uncertainties. Simple batch melting calculations with appropriate extents of melting (6% in the north and 12% in the south) 26 and assuming incompatible Mo behaviour in mantle minerals can reproduce the Mo concentrations of both the northern (~1 ± 0.25 ppm) and southern (~0.5 ± 0.25 ppm) pSMI by invoking a concentration in the mantle (0.065 ± 0.015 ppm) that is higher but within error of both the primitive (0.047 ± 0.019 ppm) 17 and averaged primitive-depleted (0.036 ± 0.020 ppm) 17,18 mantle. However, the much higher pSMI Mo concentrations in the north than the south for a given SiO2 concentration (c) are consistent with increased Mo addition during magma differentiation in the lower crust (e.g., ref. 27 ). Although it is difficult to determine whether the Mo-rich component is incorporated during any or all of lower crustal contamination (e.g., ref. 2 ), subduction erosion (e.g., ref. 5 ) or interaction with metasomatized subcontinental lithospheric mantle (e.g., ref. 6 ), MASH-type processes will always favor Mo ore-fertility in the likely case that the lower crust contributes Mo.

Supplementary Fig. 4 | Variation in fSMI of incompatible elements that are nominally fluid mobile (K2O) and immobile (Nb) during subduction, and host mineral Mg#.
Crosses along the right margin show representative 1σ uncertainties. As differentiation proceeds (including crystallization and assimilation) and Mg# decreases, incompatible elements (a, b) increase more significantly at Maipo and San Jose than to the south indicating increased assimilation of crustal melts. Overall, the large range in the concentration of highly incompatible elements over a narrow range in host Mg#, and while the magma remained in the olivine stability field for most volcanoes, demonstrates the significance of magma contamination by assimilated low-degree crustal melts. The significant span of highly incompatible trace element concentrations at a fixed host Mg# as apparent at several volcanoes reflects the buffering of major element concentrations and Fe/Mg ratios by a crystal mush at the site of crustal assimilation, likely at lower to mid crustal depths.

Supplementary Fig. 5 | Example LA-ICP-MS analyses from San Jose (a) and Antuco (b).
Analyses from San Jose and Maipo typically exhibit strong, localized S-peaks (a), whereas analyses from the other volcanoes typically have S distributed more evenly throughout the SMI signal interval (b).
Copper is typically correlated with the S peak when present.